Russian opposition leaders on Saturday accused the Kremlin of being behind the death of a towering figure of post-Soviet politics, Boris Nemtsov, as they struggled to come to grips with the highest-profile assassination of President Vladimir Putin’s 15 years in power.

Nemtsov was gunned down late Friday, steps from the Kremlin and underneath the swirling domes of St. Basil’s Cathedral — the heart of power in Russia and one of the most secure areas in the nation. The slaying of one of Putin’s most biting critics swept a wave of fresh vulnerability over those in the opposition, and some expressed new fears for their lives.

It was unclear whether the slaying would spur new support for the beleaguered opposition movement or whether it would simply be further marginalized, repressed by fear and the silencing of one of its most prominent voices.

Nemtsov’s death was a bitter bookend to the hopes that had accompanied the dashing, Western-style politician in the heady years after the breakup of the Soviet Union as he took a lead role in plunging Russia into capitalism. Now many of those reforms have been undone, with Putin taking near-absolute, personal control of the country and re-nationalizing broad swathes of the economy.

Nemtsov allies said that he had been preparing to release a report detailing evidence that Russian soldiers were fighting in Ukraine alongside pro-Russian rebels, an accusation the Kremlin has hotly denied. Two years ago, he prepared an investigation that he said uncovered a vast corruption scheme in the lead-up to the 2014 Winter Olympics in Sochi — an effort that was said to have particularly irked Putin.